The present invention relates to a hydraulic valve lift device for an internal combustion engine and, more particularly, to a hydraulic valve lift device adapted for relaxing impacts at the time of seating of the valve.
There have been proposed various types of valve lift devices for varying the valve lift timing in accordance with load imposed on the engine or running speed of the engine. In one of these known devices, which is basically identical to a known hydraulic tappet device, the oil in an oil chamber pressurized by a plunger engaging a push rod and by a lifter body is allowed to be relieved to the outside through a restriction or orifice so as to reduce the volume of the oil chamber, thereby changing the lift of the plunger.
In this type of valve lift device, the cam contour is of two dimensions, and it is not necessary to change relative positions of the cam shaft and the plunger to each other, so that the construction of the device is considerably simple. However, since the device is constructed in such a manner as to vary the valve lift by relieving the oil from the oil chamber, it is impossible to make the cam have such a cam contour involving a curvature for reducing the moving velocities of the intake and exhaust valves at the instant of seating as in cams conventionally used in engines. Therefore, a substantially large impact load acts on the valve at the time of seating to generate a large noise, thereby causing disadvantages in terms of durability of the valve mechanism.
To solve these problems, U.S. patent application Ser. No. 034,186, filed Apr. 27, 1979 assigned to the applicant of this application, discloses a hydraulic valve lift device which comprises a braking chamber defined by a flange portion formed on a plunger and the cylindrical bore of a housing, said braking chamber being adapted to be supplied with oil through a slit during upward stroke of the plunger, said slit being adapted to be decreased in its opened area during downward stroke of the plunger to reduce an amount of oil relieved from the braking chamber, so that hydraulic pressure thus increased in the braking chamber exerts a braking force on the plunger, to absorb an impact load at the time of seating of the intake or exhaust valve.
With the device of the above U.S. patent application, however, the action of a braking force is not always stable under the influence of the operating condition of the engine (for example, changes in oil temperature, running speed and so on).